1. Field of the Invention
Embodiments of the invention generally relate to a method for monitoring a process for pre-cleaning an at least partially exposed layer disposed on a substrate.
2. Background of the Related Art
Sub-quarter micron, multi-level metallization is one of the key technologies for the next generation of ultra large scale integration (ULSI). The multilevel interconnects that lie at the heart of this technology require planarization of interconnect features formed in high aspect ratio apertures, including contacts, vias, lines and other features. Reliable formation of these interconnect features is very important to the success of ULSI and to the continued effort to increase circuit density and quality on individual substrates and die.
The increase in circuit densities primarily results from a decrease in the widths of vias, contacts and other features as well as a decrease in the thickness of dielectric materials between these features. Cleaning of the features to remove contaminants prior to metallization is required to improve device integrity and performance. The decrease in width of the features results in larger aspect ratios for the features and increased difficulty in cleaning the features prior to filling the features with metal or other materials. Failure to clean the features can result in void formation within the features or an increase in the resistance of the features. Therefore, there is a great amount of ongoing effort being directed at cleaning small features having high aspect ratios, especially where the ratio of feature width to height is 3:1 or larger.
The presence of native oxides and other contaminants within a small feature contributes to void formation by promoting uneven distribution of a depositing material such as metal. Regions of increased growth merge and seal the small features before regions of limited growth can be filled with the depositing metal. Native oxides form within the features when a portion of a layer (or sublayer), such as silicon, aluminum, or copper, is exposed to oxygen in the atmosphere or is damaged during a plasma etch step. Other contaminants within the features can be sputtered material from an oxide over-etch, residual photoresist from a stripping process, leftover polymer from a previous oxide etch step, or redeposited material from a sputter etch process.
The presence of native oxides and other contaminants also can reduce the electromigration resistance of vias and small features. The contaminants can diffuse into the dielectric layer, the sublayer, or the deposited metal and alter the performance of devices that include the small features. Although contamination may be limited to a thin boundary region within the features, the thin boundary region is a substantial part of the small features. The acceptable level of contaminants in the features decreases as the features get smaller in width.
Pre-cleaning of features to remove native oxides and other contaminants has become increasingly utilized to prepare surfaces for barrier layer or metal deposition. One process for removing native oxides and other contaminants from polysilicon, copper and metal surfaces is described in U.S. Pat. No. 6,107,192, issued Aug. 22, 2000 to Subrahmanyan et al., which is hereby incorporated by reference in its entirety. This process, which may be performed in a REACTIVE PRE-CLEAN(trademark) II process chamber, available from Applied Materials, Inc., of Santa Clara, Calif., generally includes a first cleaning step and a second reducing step. The cleaning step features a soft plasma etch using a reactive gas such as oxygen, a mixture of CF4/O2, or a mixture of He/NF3, wherein the plasma is preferably introduced to the chamber from a remote plasma source. The remaining native oxides are then reduced in the second step by treatment with a hydrogen comprising plasma.
Typically following the first or both pre-cleaning steps, the features can be filled with metal by available metallization techniques which typically include depositing a barrier/liner layer on exposed dielectric surfaces prior to deposition of aluminum, copper, or tungsten. The pre-cleaning and metallization steps can be conducted remotely or preferably on integrated processing platforms, such as the family of ENDURA(copyright), PRODUCER(copyright) and CENTURA(copyright) processing platforms, all available from Applied Materials, Inc., of Santa Clara, Calif.
As the removal of native oxide and other contaminants directly enhance device performance, monitoring of the effectiveness of the pre-clean process is advantageous to ensure robust process chamber performance. Typically, pre-clean processes are monitored by taking reflectivity measurements of the exposed layer on the substrate. As the presence of oxides and other contaminants on the oxide layer directly changes the reflectivity of the exposed layer, the measured reflectivity is an indicator of the presence of native oxides or other contaminants on the exposed surface of the substrate. Reflectivity is typically measured in pre-clean processes using optical devices. Generally, a beam of light is reflected off the substrate surface to the sensor. As the reflectivity of the exposed film is indicative of the composition of the film (i.e., whether contaminants or native oxides are residing on the surface) the cleanliness of the film can be determined.
However, when using optical devices to measure reflectivity of a material, care must be taken not to introduce measurement errors. For example, focal distance between the sensors and the substrate, which are easily disturbed, must be maintained. This results in a need to frequently calibrate the measurement system. Additionally, the beam generator and sensor are sensitive to contamination on their lenses. Moreover, the surface roughness of the film, which could be changed by the pre-clean process, may affect the reflectivity by changing the refraction characteristics of the surface. Thus, as the demand for smaller feature sizes increases the importance of the elimination of contaminants and native oxides from the exposed surfaces, a more robust measuring system is needed to ensure robust and efficient pre-cleaning processes.
Therefore, there is a need for an improved method for pre-cleaning an at least partially exposed layer disposed on a substrate.
In one aspect of the invention, a method for monitoring a process of removing native oxides from an at least partially exposed layer disposed on a substrate is provided. In one embodiment, a method for monitoring a process of removing native oxides from an at least partially exposed layer disposed on a substrate includes disposing the substrate in a process chamber, exposing the at least partially exposed layer to a reactive pre-clean process and measuring a sheet resistance of the exposed layer.
In another embodiment, a method for monitoring a process of removing native oxides from an at least partially exposed conductive layer disposed on a substrate includes disposing the substrate in a process chamber, exposing the at least partially exposed conductive layer to a reactive pre-clean process that comprises an oxide reduction step, removing the substrate from the process chamber, contacting the at least partially conductive layer with one or more contact members, measuring a sheet resistance of the at least partially exposed conductive layer between the contact members, and comparing the measured resistance to a known value.
In yet another embodiment of the invention, a method for monitoring a process of removing native oxides from an at least partially exposed conductive layer disposed on a substrate includes depositing a copper seed layer on a sample substrate in a first chamber, exposing the copper seed layer to a reactive pre-clean process in a second chamber to remove native oxides from the copper seed layer, transferring the sample substrate from the second chamber to a metrology device, measuring the sheet resistance of the conductive layer, and comparing the measured sheet resistance to a known value.